The present invention relates to an interface for allowing a computer to communicate with a peripheral device. More particularly, the invention relates to apparatus which enables a host computer, which inputs and outputs data in a serial format, to communicate with an external machine, such as a postage meter, which receives and transmits data in an echoplex format.
In using computers to control processes or other machinery, it is necessary to provide some way to able the computer to communicate with the apparatus to be controlled. Apparatus which is used to effect such communication is generally referred to as an interface. The interface must handle data transferred to and from the computer in the manner in which the computer outputs the data or expects to receive the data. Similarly, the apparatus being controlled by the computer will output data and expect to receive data in a particular format. The interface must provide for the desired communication between the computer and external apparatus, and if necessary must convert the data from the computer so that it will be understood by the external apparatus, and convert data from the external apparatus so that it will be understood by the computer.
Often, the external apparatus controlled by one computer (the "host computer") will be another computer, or a machine which incorporates its own computer. Where the host computer and the external apparatus use different communication schemes, an interface is required to convert data from one so that it is intelligible to the other, and vice versa.
One type of communication scheme often used by host computers is known as serial data communication. In serial communication, data is transmitted and/or received one bit at a time over a single wire pair. Sometimes, one wire is used to transmit serial data and another wire is used to receive serial data.
Another communication scheme sometimes used by computers is known as parallel communication. In parallel communication, a plurality of data bits are transmitted simultaneously along as many wires as there are bits to be communicated. Typically, data will be transmitted in 8-bit "bytes", in which case eight separate lines are utilized for the parallel communication of each of the 8 bits simultaneously.
Another type of communication, which has been used in electronic postage meters, is known as echoplex communication. Echoplex communication is a form of serial communication, wherein serial messages are asynchronously transmitted and received. The format of the messages, and the timing of the bits in different units is precisely set, however, to ensure that messages may be sent and received without the necessity for synchronizing each communicating computer. In addition, in echoplex communication, upon receipt of the first bits of a message from a transmitting unit, the received bits are retransmitted by the receiver back to the transmitter for comparison. This comparison enables the transmitter to determine if any errors have occurred in its transmission, or the receiver's reception, of data. Thus, with echoplex communication, the correctness of each message sent and received is verified within a minimum period of time following the complete message transmission.
The theory of echoplex communication, and in particular, its application to an electronic postage meter, is disclosed in U.S. Pat. No. 4,301,507, entitled "Electronic Postage Meter Having Plural Computing Systems", Issued Nov. 17, 1981, and assigned to the assignee of the present invention. As discussed in that patent, it is sometimes desirable to control the functions of an electronic postage meter from a remote location. In order to accomplish such control, an interface connector is provided in the electronic postage meter, for connection to the remote controller. Thus, for example, connection of an external device, such as an electronic scale, to the postage meter can be made to more fully automate the mailing process.
In order to provide additional opportunities for the remote control of electronic postage meters, an interface is required which will enable the remote controller (e.g., a host computer) to communicate with the postage meter to be controlled. Since the host computer will generally transmit and expect to receive data in a standard serial format, whereas the postage meter will transmit and expect to receive data in an echoplex serial format, the interface will have to provide for the conversion from one form of serial data to echoplex data, and vice versa. Such an interface should operate in real time, such that data from the postage meter is received as it is transmitted, and data from the host computer is received by the postage meter as it is transmitted.
Further, it would be advantageous if such an interface were transparent to both the postage meter and the host computer When an interface is referred to as transparent, it operates such that the apparatus which the interface couples together does not realize that the interface is present. In other words, a postage meter connected to host computer through a transparent interface will think that it is communicating directly with the host computer, and the host computer will think that it is communicating directly with the postage meter.
The present invention relates to such a universal real time transparent asynchronous serial/echoplex converter.